1. Field of the Invention
The present invention relates to a process for continuously producing a hydrogenation product of an aromatic carboxylic acid having a high melting point and hardly soluble in solvents in the presence of a solid catalyst.
2. Description of the Prior Art
In general, aromatic carboxylic acids and, in particular, aromatic dicarboxylic acids have high melting points and small solubilities in various solvents. On the other hand, the reaction for producing cyclohexanedicarboxylic acids and cyclohexanedimethanols such as 1,4-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedimethanol and 1,3-cyclohexanedimethanol from aromatic dicarboxylic acids used as the raw material such as terephthalic acid or isophthalic acid by bringing the raw material into contact with hydrogen is, in general, conducted in a solvent in the presence of a solid catalyst.
Heretofore, in many cases, the above reaction is conducted in accordance with a batch process in which the reaction is conducted at a specific temperature after a raw material and a catalyst are placed into a reactor in combination with a solvent and the obtained reaction product is taken out of the reactor.
For example, a process for producing 1,4-cyclohexanedicarboxylic acid comprising forming 1,4-cyclohexanedicarboxylic acid by treating terephthalic acid with hydrogen at a temperature of 110xc2x0 C. or higher and 180xc2x0 C. or lower using water as the solvent in the presence of a catalyst containing palladium and ruthenium and treating the obtained reaction liquid by solid-liquid separation at a temperature in a specific range is described in Japanese Patent Application Laid-Open No. Showa 58(1983)-198439. Although it is described that the process can be conducted in accordance with any of the batch process, the semi-continuous process and the continuous process (lines 15 and 16 at the upper right side of page 3), the batch process of the reaction alone is shown in the examples and no descriptions can be found on the continuous process of the reaction.
A process for producing 1,4-cyclohexanedimethanol by hydrogenation of terephthalic acid in an aqueous medium in the presence of a catalyst containing ruthenium and tin as the components is described in Japanese Patent Application Laid-Open No. 2000-7596. Although it is described that the process can be conducted in accordance with any of the continuous process and the batch process (lines 2 and 3 at the right side of page 3), the batch process of the reaction alone is shown in the examples and no specific descriptions can be found on the continuous process of the reaction.
The reaction in accordance with the batch process has a problem in that the time is consumed ineffectively for procedures which substantially do not contribute to the reaction such as placing raw materials, elevating the temperature to the reaction temperature and taking out the reaction product and the productivity of the reactor is poor. The batch process has a further problem in that, when a catalyst is used repeatedly, separation of the catalyst and the reaction product takes time and energy and the activity of the catalyst decreases rather rapidly due to a change in the temperature.
Therefore, it is desirable that the hydrogenation of an aromatic carboxylic acid is conducted in accordance with the continuous process. As the process for continuously hydrogenating an aromatic carboxylic acid, a process in which an aromatic carboxylic acid is dissolved in water in the form of a salt of an alkali metal and supplied to a fixed bed reactor packed with a catalyst (Japanese Patent Application (as a national phase under PCT) Laid-Open No. Heisei 7(1995)-507041) and a process in which a dialkyl ester of terephthalic acid is used as the raw material for the hydrogenation (Japanese Patent Application Laid-Open No. Heisei 10(1998)-45646) have been known. In these processes, the aromatic dicarboxylic acid is continuously hydrogenated after being converted into a compound having a greater solubility or a lower melting point.
However, the above processes has problems in that additional steps are required in the production process to convert the aromatic carboxylic acid into a compound having a greater solubility or a lower melting point and that auxiliary materials such as an alkali metal compound or an alkyl alcohol have to be used. Therefore, a process for directly hydrogenating an aromatic carboxylic acid in accordance with a continuous process has been desired.
Since ordinary organic compounds have greater solubilities at elevated temperatures, the reaction may be conducted at a higher temperature so that a sufficient solubility can be obtained. However, the reaction at a high temperature has problems in that the yield of the object compound decreases due to decomposition of the raw material and the reaction product and an increase in the amount of byproducts and that the activity of the catalyst decreases rapidly.
For example, when a cyclohexanedicarboxylic acid or a cyclohexane-dimethanol is produced by bringing terephthalic acid or isophthalic acid into contact with hydrogen using water as the solvent, the reaction conducted at a temperature at which the above benzenedicarboxylic acid can be completely dissolved into an industrially suitable amount of water, which is, specifically, a temperature exceeding 200xc2x0 C., results in a decrease in the yield of the reaction and deterioration in the purity of the reaction product due to decomposition of the raw material and excessive hydrogenation.
The present invention has, under the above circumstances, an object of providing a process for efficiently producing a hydrogenation product of aromatic carboxylic acid of the object compound by hydrogenation in accordance with a continuous process in the presence of a solid catalyst using as the raw material an aromatic carboxylic acid having a high melting point and a low solubility in solvents, which can hardly be brought into an efficient reaction in accordance with conventional processes.
As the result of intensive studies by the present inventors on the process for efficiently conducting hydrogenation of the above aromatic carboxylic acid in accordance with a continuous process, it was found that the continuous reaction could be conducted at the most suitable reaction temperature in a fixed bed reactor when the reaction liquid was used at least as a portion of the solvent for dissolving the raw material by recycling the reaction liquid and the inside of the reactor was brought to a condition such that substantially the entire amount of the raw material was dissolved. The present invention has been made based on the knowledge.
The present invention provides a process for producing a hydrogenation product of aromatic carboxylic acid which comprises continuously producing the hydrogenation product of aromatic carboxylic acid by hydrogenating an aromatic carboxylic acid having a melting point of 250xc2x0 C. or higher in a solvent in a presence of a solid catalyst, wherein the hydrogenation is conducted in a condition such that substantially an entire amount of the aromatic carboxylic acid of a raw material is dissolved in the solvent by recycling a portion of a reaction liquid taken out of a reactor into the reactor.